Single crystals of the sulfur-containing DNA base analog 6-methylmercaptopurine have been prepared and irradiated by X rays. Electron spin resonance techniques have been used to study the radiation-induced radicals at room temperatures. Four radical species (I to IV) were observed to be stable at room temperature. None of these radicals showed resolvable hyperfine structure, but three had large g-value variations about average values of I, 2.026; II, 2.012; and III, 2.0078. These three radical species are attributed to π-electron monosulfide radicals formed by rupture of the${\rm S}-{\rm CH}_{3}$ bond. Radical species IV is believed to be a hydrocarbon-centered radical with very little spin density on hydrogen or nitrogen atoms. These results indicate that the formation of the hydrogen-addition radical on the purine ring, found in other irradiated purines at 300 K, may be hindered by sulfur substituents in purines.

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